A conventional sheet stacker will be described by referring to FIG. 1. A corrugated cardboad web 101 which has been continuously manufactured through various preceding steps is cut off into several thinner sheets in the direction of advancement and, thereafter, widthwisely cut off by means of a cutter 102 at intervals of a predetermined length into corrugated cardboard sheets 103. The sheets 103 are discharged from a cuter outlet by a conveyor 104 to a shingling conveyor 105 which is driven at a lower speed than the former conveyor 104, so that the singled or overlapped sheets (in the form of stacked roofing slates) are fed onto a transfer conveyor 106. A plurality of braking members such as brushes, leaf springs or free rollers are disposed above the shingling conveyor 105 to restrain advance of the sheets. Because the sheets are cut off by means of the cutter to any desired length usually in a range of 500-5000 mm, the braking members are manually adjusted between their operative and inoperative modes depending on the length of the sheets.
The sheet 103 is discharged onto a sheet stacking table 107 through the transfer conveyor 106. More specifically, the discharged sheet 103 strikes against a front plate 109 and drops downward to be stacked on the sheet stacking table 107 in sequential order. The sheet stacking table 107 is driven up and down by a motor 110 through sprockets 111, 112 and chains 113, 114, 115. An upper end level of the sheets stacked on the table 107 is detected by a photoelectric tube 108. When the sheets 103 interrupt an optical path of the photoelectric tube 108, the motor 110 is driven and, when they do not interrupt the optical path, the motor 110 is stopped. Thus, the motor 110 is controllably driven so that the distance a that the sheets fall from the transfer conveyor 106 is kept substantially constant. Designated at 116 is a limit switch which is actuated upon downward movement of the table 107 for stopping the motor 110.
In such a conventional sheet stacker, the braking members must be manually moved up and down for each order change to vary a length of the sheets 103. This manual setting is troublesome and often not in good timed relationship with the order change. If not in good timed relationship, the sheets just after change in length are not favorably braked, with the result that they may be disordered, folded or travel too fast and hence became jammed.
The corrugated cardboard sheet 103 to be manufactured is divided into several types having different thicknesses of 3 mm, 5 mm and 9 mm, for example, depending on the size of corrugations, and the number of sheets discharged from the transfer conveyor 106 onto the table 107 is largely varied in accordance with the manufacturing speed and length of the sheets. Meanwhile, the descent speed of the sheet stacking table, i.e., a rotational speed of the motor 110, must be large enough to be capable of following handling the maximum capacity of stacked sheets. Since the descent speed of the table is so set in the above sheet stacker, the table descent speed becomes too large for the normal amount of stacked sheets and descending of the table can not be stopped with fine enough control, thus resulting in a larger fall distance a. With the increased fall distance a, the dropping sheets are more largely disordered so that they are not stacked on the table in a neat order but instead are stacked in a random state. Such a random state gives rise to the problems that the stacked sheets are liable to break and the projecting portions of the sheets may be damaged, when transferred to the next step, and that handling of the sheets in the next step becomes difficult and automization of the handling is hampered due to the resulting difficulty.
Furthermore, the corrugated cardboard sheets manufactured by a corrugation machine include various types of defective sheets which are caused through the manufacturing process as a result of failed bonding, curvature, worn-out edge, stains, scratches, etc. If these defective sheets are mixedly stacked in with the good sheets at the stacker section as the final step of a corrugation machine, a difficulty is encountered in operation such as fabricating the sheets into boxes, or putting them into print. Accordingly, the defective sheets must be removed during the operations of a corrugation machine. Heretofore, the defective sheets have been visually checked and then withdrawn by an operator. This method is favorably effective for a small amount of defective sheets. According to circumstances, howevr, a large amount of defective sheets may be produced. In such a case, it is very troublesome to remove the defective sheets by hands and the machine must be often stopped for removal thereof. To cope with this, there has also been conceived an apparatus for automatically removing the failed sheets. But, because of the needs of detecting the varius types of defective sheets as well as very high-graded detection techniques, the conceived apparatus is practically infeasible from both technical and economic standpoints.